This invention relates to a new low-heat, high-performance concrete.
Many engineering applications require high performance concrete having high strength and hardness and low permeability to air and water. Standard High Performance Concrete ("SHPC") exhibits such qualities having a 28 day unconfined compressive strengths (.sigma..sub.c) of 70 MPa or more.
SHPC relies on high Portland cement content in the range of 450-550 kg/m.sup.3 (or about 20% by weight) to achieve high strength. Portland cement is an unhydrated mixture of oxides and sesqui-oxides of compounds of calcium, silicon and aluminum combined with trace elements and compounds. The four major compounds in Portland cement are C.sub.3 S, C.sub.2 S, C.sub.3 A and C.sub.4 AF where C is CaO, S is SiO.sub.2, A is Al.sub.2 O.sub.3 and F is Fe.sub.2 O.sub.3. The silicates C.sub.3 S and C.sub.2 S are generally ascribed as the compounds from which materials such as concretes, mortars and grouts, that are formed when the cement is hydrated, derive their desirable mechanical and engineering properties. When hydrated, the silicates form C--S--H (tobermorite) gels and CH (portlandite), where H is H.sub.2 O.
The hydration of cement produces an exothermic chemical reaction. The main generation of heat, which is initially retarded by gypsum in the mixture, begins at about 12 hours after mixing with water, reaches a maximum rate between about 24 and 48 hours after mixing, and then begins to decrease. Empirical relationships indicate that, when perfectly insulated against heat loss, for each 100 kg of cement per cubic meter of concrete, the heat of hydration will increase the temperature of the concrete by between about 8.degree. and 12.degree. C.
As the heat produced in concrete is proportional to the cement content, SHPC which relies on high cement content undergoes considerable heating during curing. As a result, SHPC is generally unsuitable for mass concrete structures. During hydration of mass concrete structures, the rate of heat generation far exceeds the rate of dissipation to the surroundings producing a non-linear temperature distribution across the structure. This induces tensile stresses which can cause surface cracking. In addition, the volume change associated with the temperature change as the heat is dissipated also induces tensile stresses leading to continuous splitting cracks.
High rates and quantities of heat generation in mass structures also leads to lower final strength. The lower final strength of concrete cured at a high temperature has been attributed to accelerated hydration which results in nonuniformly distributed C--S--H hydration products formed closely around the cement particles, and to internal stresses and microcracking.
Some of the effects of high rates and quantities of heat generation can be mitigated by special procedures such as the use of cooled water and aggregates, the inclusion of ice during the preparation of the concrete, and liquid nitrogen cooling of the fresh concrete. These procedures increase costs and can produce technically undesirable results.
It is known to substitute silica fume for a portion of the Portland cement in order to reduce the heat of hydration and enhance the properties of SHPC. Silica fume, a waste product from the ferro-silicon manufacturing process, consists of amorphous silicates with a mean equivalent spherical diameter of about 0.25 .mu.m. Silica fume reacts with the CH liberated during the hydration of Portland cement to form C--S--H. The total heat released by this pozzolanic reaction is double that released during the hydration of Portland cement. However, the pozzolanic reaction proceeds at a slower rate than the cement hydration reaction and as a results, the heat from the pozzolanic reaction does not build up in, and increase the temperature of, the concrete as much as the heat of hydration from the more rapid cement hydration reaction.
While reduced heat build-up and enhanced properties can be realized by the addition of silica fume, high addition levels of 30% by dry mass replacement of cement are reported by Malhotra et al. in Condensed Silica Fume in Concrete, 1987, CRC Press, Inc. to cause increased shrinkage. Accordingly, silica fume addition levels are generally restricted to between 10 and 15%.
It is also known to add quantities of inert fillers to mixtures of Portland cement and silica fume to increase the strength of the mixture. In U.S. Pat. Nos. 4,482,385, 4,505,753 and 4,780,141, cementitious compositions are disclosed which contain Portland cement, silica fume and other ingredients including fine aggregate, preferably a crystalline silica having a particle size below 5 microns. Although these compositions contain substantial quantities of silica fume and inert filler, they rely primarily on the conventional understanding that high strength and enhanced properties are achieved through low water to cement ("W/C") and low water to cementitious materials ("W/CM") ratios. In particular, the W/C ratios in the preferred compositions of the '385, '753 and '141 patents are 0.24, 0.24 and 0.25 respectively and the W/CM ratios are 0.27, 0.27 and 0.28 respectively. Furthermore, the cement to silica ratios of these compositions are about 1:0.62, 1:0.63 and 1:0.60. Such proportions of cement can be expected to produce substantial heats of hydration approaching that of conventional Portland cement based concretes.